U.S. Pat. No. 4,189,675 proposes a satellite communications method and apparatus for communicating with mobile users using a satellite in a predetermined orbit.
EP 0562374 and EP 0568778 are believed to describe the "Iridium" proposed satellite cellular mobile communication system.
An alternative description of the "Iridium" proposals is given in the paper "The Iridium (TM) system personal communications anytime, any place" J. E. Hatlelid and L. Casey, Proceedings of the Third International Mobile Satellite Conference IMC 93, Jun. 16-18 1993, pages 285-290.
An alternative proposed satellite cellular system is described in "The Globalstar Mobile Satellite System for Worldwide Personal Communications", R. A. Weideman, pages 291-296 of the Conference Proceedings mentioned above. Two alternative access network schemes, invented by the author of that paper, and believed to have been proposed for use in the proposed Globalstar system, are described in EP 0536921 and EP 0506255.
GB-A-2295296 and WO-A-96/16488 describe a satellite communications network and in particular the ground segment thereof.
Various terrestrial digital cellular communications systems are known or proposed. Of these, the GSM system is widely known.
In the GSM system, as in other cellular systems, in order to be able to direct incoming calls to a user, it is necessary to be aware of the location of the user (so called "mobility management"). In GSM, this is achieved by the provision of two layers of databases; so called "home location registers" (HLRs), and so called "visiting location registers" (VLRs), and by registration and location updating signalling processes.
Subscriber data on a given user and/or user terminal is stored in a specific HLR for that user. Each mobile switching centre (MSC) associated with a particular geographical area has an associated VLR, in which are temporarily stored details necessary for call management of all users currently thought to be within the area of that MSC.
Initially, a mobile terminal scans the broadcast common control channels (BCCHS) originating from all base stations (BSCs) within its reception, and attempts to register with one. The registration takes the form of an exchange of validation data, as described in "Security aspects and the implementation in the GSM-system", Peter C. J. van der Arend, page 4a, Digital Cellular Radio Conference (DCRC) Conference Proceedings, Oct. 12-14 1988, published by Deutche Bundespost, France Telecom and Fernuniversitate.
Specifically, the mobile terminal identifies itself by transmitting its identity number (IMSI), and the mobile switching centre (MSC) receiving the identification signal from the mobile signals the home location register (HLR) of the mobile and receives authentication data which is stored in the visiting location register (VLR).
The MSC then sends an authentication request signal to the mobile together with a random number, and the mobile uses the random number together with an individual stored subscriber authentication key to calculate a signed response (SRES) which is transmitted back to the MSC.
The MSC then compares the signed response with the security data including a signed response which was supplied by the HLR and, if the two match, the mobile terminal is registered as being within the area of the VLR and MSC. On registration, the identity of the VLR is stored in the HLR for the mobile terminal.
Subsequently, when incoming calls arrive for the mobile terminal, the HLR is accessed to determine the VLR where the mobile is registered and calls are then routed to the MSC associated with that VLR.
Even in idle or "sleep" mode, mobile terminals will continually or periodically scan the broadcast control channels (BCCHs) which they can receive. Each BCCH of a cell carries, amongst other data, a location area identifier (LAI) indicating an individual cell or a group of cells within a certain area. When a new location area indicator is detected (indicating that the mobile terminal has moved into a new area), the mobile terminal transmits a location update request indicating the new LAI.
The exchange of authentication data is repeated, and if the mobile terminal is authenticated the new location area indicator is written into the VLR.
Thus, the VLR continually maintains an indication of which area (and, more specifically, which cell) the mobile terminal is within.
A mobile terminal may also move between the areas of two different VLRs.
Similar issues will arise in relation to satellite communications systems; see, for example, the paper "Study on network issues of medium earth orbit satellite communications systems"; Araki et al, Proceedings of the Third International Mobile Satellite Conference IMSC 1993 (JPL publication 93-009), pages 529-534, published by Jet Propulsion Laboratories (1993). In that paper it is described how either each land earth station may issue a location area identifier signal which is carried by spot beams of satellites within the area, or each spot beam of each satellite may carry a location area identifier.
In the GSM system, there is also a signalling procedure to secure periodic registration from mobile stations, to maintain information on the status of mobile stations.
The mobile terminal performs periodic reregistration or location updating using a timer with a time out over a range of between 6 minutes and 25 hours 30 minutes, the time out being set in accordance with a parameter which is optionally transmitted in the BCCH. The timer value is reset after each signalling activity on the radio path. The current timer value is stored in non-volatile memory when the mobile station powers down. On restoring power, the time starts running again from the value stored in the memory.
The frequency of such registration updates affects the accuracy with which the position of the mobile terminal is known, which is important in setting up calls to or from the user. However, every re-registration or location update uses user terminal and satellite battery power, both of which are potentially scarce resources. Furthermore, the overhead of signalling traffic uses scarce network channel resources without generating revenue.
Accordingly, the present invention provides a method of operating a user terminal to provide mobility management in a communications system in which the user terminal registers data concerning its location in a remote management store, the method including: monitoring signals received from the communications system in relation to predetermined criteria, and upon the monitored signals not meeting the criteria for more than a predetermined time (T2), transmitting from the user terminal, signals for updating the location of the user terminal registered in the management store, when said monitored signals again meet said criteria.
A corresponding terminal is also provided.
Other aspects and preferred embodiments of the invention are as described or claimed hereafter, with advantages which will be apparent from the following.